1. Field of the Invention
The present invention relates to a telephoto zoom lens consisting of four lens groups, namely positive, negative, positive and positive lens groups arranged in the named order from the object side.
2. Description of the Prior Art
The above-mentioned type of zoom lens is known in the art but has many drawbacks.
In the known zoom lens, the magnification change of the image is achieved by moving the first and fourth lens groups together in the direction toward the object side while correcting the image position by the third lens group. Herein, the first lens group means the lens group nearest to the object side and the fourth lens group means the lens group nearest to the image side. One of the important problems involved in this type of the known zoom lens is the variation of aberrations, especially that of coma. With the movement of the fourth lens group during the magnification change the exit angle of oblique light beam varies and therefore the off-axial aberrations vary very much with magnification change. Because of it, at the telephoto side there is produced a great deal of internal coma which is very difficult to correct well.
In order to compensate the variation of image plane caused by magnification change it has been proposed to provide the negative lens component at the object side of the second lens group with a cemented surface having a positive refractive power. Such a modification of the zoom lens is disclosed, for example, in Japanese laid-open Patent Application No. 143,313/1983. However, this modified known zoom lens also has some disadvantages. With the arrangement, it is no longer possible to increase up the refractive power of the second lens group, which is against the general desire for compact structure of the whole system. In addition, it renders the coma excessive at the wide angle side. Consequently, the image forming performance is remarkably degraded by the large variation of coma with the magnification change.
Another important problem of the known zoom lens is the variation of F-number with magnification change. Since the fourth lens group has to be moved a large distance for magnification change, the variation of F-number is inevitable in the known lens system. Especially in those zoom lenses which have high magnifications, the variation of F-number is remarkedly large. Therefore, the freedom available for the correction of aberrations is almost used out for another purpose of reserving the brightness on the wide angle side. On the telephoto side, the lens system is necessarily made as a dark lens. This disadvantage may be overcome by continuously changing the opening of aperture stop during magnification change. However, it needs a complicated mechanical structure which is entirely against the general desire for the reduction of size and manufacturing cost of such zoom lenses.